How can I provide refund to the customer from my application instead of going to authorize.net

my application is written in classic asp

we are using two methods echeck and credit card

please tell how to refund for both of them are the refund will be same or will be in different way

we are not storing credit card on our server so i do not have last four digits of card,

but we have

transaction id

x_login

x_tran_key